1. Field of the Invention
This invention relates to the combination of certain energy sensitive organic dyes in photothermographic elements, compositions and processes which do not require the presence of a silver halide to provide a developed image using a reasonably short exposure time. By a selective exposure technique either a negative or a positive image can be obtained. In one of its aspects, this invention relates to photothermographic elements containing said combination of dyes. In another of its aspects it relates to a photothermographic composition containing the described combination of dyes. A further aspect relates to a method of developing a latent image in the described photothermographic elements to a negative image by overall heating or to a positive image by imagewise exposure to ultraviolet light and subsequent exposure to a tungsten lamp followed by overall heating.
2. Description of the State of the Art
It is known in the art to provide photothermographic materials containing silver halide which can be imagewise exposed and heat developed to form negative photographic images. Such photothermographic materials after exposure are heated overall to provide a developed negative image in the absence of separate processing solutions. Reducing agents which have been employed with silver halide in photothermographic materials are described, for example, in Belgian Pat. No. 765,452, issued May 28, 1971; Belgian Pat. No. 765,602, issued May 28, 1971; Belgian Pat. No. 765,601, issued May 28, 1971; Belgian Pat. No. 766,590, issued June 15, 1971; Belgian Pat. No. 766,589, issued June 15, 1971; Belgian Pat. No. 772,371, issued Oct. 15, 1971; U.S. Pat. No. 3,152,904 of Sorenson et al., issued Oct. 13, 1964; U.S. Pat. No. 3,152,903 of Sorenson et al., issued Oct. 13, 1964; U.S. Pat. No. 3,457,075 of Morgan et al., issued July 22, 1969; U.S. Pat. No. 3,392,020 of Yutzy et al., issued July 9, 1968 and British Specification No. 1,161,777, published Aug. 20, 1969. The use of photographic silver halides, however, to get high contrast film in the prior art is undesirable. The high expense and shortage of silver has resulted in a continuing search for substitutes. However, at this time, no material has been found to provide high contrast at reasonable exposure times.
One method of developing photothermographic materials without a silver halide component is described in Lokken U.S. Pat. No. 3,409,438. This method entails the use of certain compounds with reducing agents to form a negative image after heat development. These photothermographic elements are not capable of forming high contrast images after being exposed for short periods of time or of producing either negative or positive images depending upon the exposure technique used.
The use of certain nitrobenzylidene dyes as impregnants for support materials for photographic elements containing silver halide is described in Belgian Pat. No. 788,279.
The use of dyes containing a heterocyclic nitrogen atom which is substituted by a member selected from the group consisting of an alkoxy group and an acyloxy group as precursors for physcial development is described in U.S. Pat. No. 3,649,275, issued Mar. 14, 1972 to McNally et al.
A non-silver-halide containing photothermographic element has not been found that is capable of being exposed for a short time and subsequently heat developed to a high contrast negative or positive image.
Accordingly, there has been a continuing need to provide improved photothermographic elements, compositions and processes that do not contain silver halide and which provide the high contrast between Dmax and Dmin images with a minimum of exposure time and an option of producing negative or positive images.